This invention relates to liquid-level gauging.
The invention is more particularly concerned with ultrasonic liquid-level gauging sensors and systems.
Ultrasonic liquid-level sensors utilize the fact that ultrasonic vibrations travel freely in a liquid but are rapidly attenuated in air or other gas. If an ultrasonic transducer is mounted on the base of a liquid reservoir so that it directs energy up towards the liquid/air interface, the energy will be reflected back down to the transducer by this interface. By measuring the time taken between transmission and reception of an energy pulse, it is possible to measure the distance between the transducer and the liquid/air interface and, from this, the depth of liquid.
It is common practice for ultrasonic transducers of this kind to be mounted at the lower end of a tube that extends from the bottom to the top of the liquid reservoir. The tube is open at the bottom so that liquid fills the tube to the same depth as in the reservoir outside the tube. The tube serves several purposes. It helps isolate the transducer from other sensors or sources of interference. It also confines the ultrasonic beam, so that it is directed only at the region of the liquid surface directly above the transducer. Furthermore, the tube produces within it a region of liquid surface that is substantially damped of waves.
Another advantage arising out of the use of the tube is that it is easy to provide a reference height, by mounting some form of reflector at a known height within the tube. In this way, the transducer will receive a reflection from the liquid surface and one from the reference reflector against which the liquid height can be calibrated. This enables the ultrasonic gauging system to compensate for different liquids having different acoustic propagation properties and for temperature variations which can affect ultra-sound propagation. An example of an ultrasonic probe having a tube of this kind is described in, for example, EP 0106677. In this previous arrangement, the reflected pulse from the reflector below the liquid level is used to provide an average indication of the velocity of sound in the fluid below that reflector. This indication of the average velocity is used, in conjunction with the time of travel of pulses reflected from the liquid surface, to calculate the fluid height. Although this does take into account variation in acoustic propagation of fluids, it does not produce accurate results where there is considerable stratification of the fluid. In aircraft applications, for example, the fuel remaining in a tank after a flight may be at a very low temperature. On refuelling, with warmer fuel, this will lie above the cold fuel and will have very different acoustic propagation properties. Taking the average of the acoustic velocity within the fuel below the uppermost submerged reflector will not, therefore, necessarily give a very accurate indication of the acoustic propagation properties for the fuel above the reflector.